Self-Assembled Morphologies of Polystyrene-block-poly(ethylene oxide)/1-Ethyl-3-methylimidazolium Thiocyanate Membranes by Mesoscopic Dynamics Simulation

The stability of gas separation membranes cast from diblock copolymer/ionic liquid (IL) blends depends on the resulting self-assembled morphologies of the cast films. Therefore, controlling the copolymer/IL morphology by tuning parameters such as IL loading and copolymer block size ratio is essential to prevent the membrane from leaching out the IL at high transmembrane pressures. In the present study, we used the dynamic mean-field density functional method to investigate the self-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymers in 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]) at different PS:PEO block size ratios and IL loadings (10–90 vol %) at room temperature (298 K). The IL was observed to be either confined by the hydrophilic PEO phase (designated as IL-PEO) due to strong IL-PEO interactions or yield a separate partially or fully encapsulated microphase (IL-Micro). The copolymer morphologies observed herein were lamellar (L), cylindrical (C), body-centered cubic (BCC), and spherical micelle (S). The dominant copolymer/IL morphology on the ternary phase diagram was L/IL-PEO, which formed at medium loadings of the three components (40 vol % < PS < 80 vol %, PEO < 90 vol %, and IL < 50 vol %). The IL-Micro morphologies appeared as transition phases to the IL-PEO phases, typically at low IL loadings and PS:PEO block size ratios. We also investigated the morphology evolution of select copolymer/IL compositions. Overall, the G, L, and C copolymer morphologies were observed at low to medium IL loadings, while the BCC and S copolymer morphologies appeared at higher IL loadings. The potential applications of these self-assembled morphologies could be further explored by investigating the role of electrostatic interactions and varying the types and loadings of ILs, as well as the type of the diblock copolymers, to discover new membrane systems with unique properties for gas separations.